I. Field
The subject technology relates generally to communications systems and methods, and more particularly to systems and methods that determine timing and frequency information in an OFDM system by applying matched filtering functions to detect received pilot symbols, where complex outputs are acquired over a time period and sampled to determine timing and frequency information.
II. Background
An air interface specification defines FLO (Forward Link Only) technology that has been developed by an industry-led group of wireless providers. The basic signal unit for FLO™ transmission is an Orthogonal Frequency Division Multiplexing (OFDM) symbol that consists of 4642 time-domain base-band samples called OFDM chips. Among these OFDM chips are 4096 data chips. The data chips are cyclically extended on each side, with 529 cyclically extended chips preceding the data portion and 17 following the data portion. To reduce the OFDM signal's out-band energy, the first 17 chips and the last 17 chips in an OFDM symbol have a raised cosine envelope. The first 17 chips of an OFDM symbol overlap with the last 17 chips of the OFDM symbol that precede them. As a result, the time duration of each OFDM symbol is 4625 chips long.
Before transmission, FLO data is generally organized into super frames. Each super frame has one second duration. A super frame generally consists of 1200 symbols (or variable number of OFDM symbols based on the bandwidth being used) that are OFDM modulated with 4096 sub-carriers. Among the 1200 OFDM symbols in a super frame, there are: Two TDM pilot symbols (TDM1, TDM2); One wide-area and 1 local identification channel (WIC and LIC) symbols; Fourteen OIS channel symbols, including four Transitional Pilot Channel (TPC) symbols; A variable number of two, six, 10, or 14 PPC symbols for assisting with position location; and Four data frames.
Time Division Multiplexing (TDM) Pilot Symbol 1 (TDM1) is the first OFDM symbol of each super frame, where TDM1 is periodic and has a 128 OFDM chip period. The receiver uses TDM1 for frame synchronization and initial time (course timing) and frequency acquisition. Following TDM1, are two symbols that carry the wide-area and local IDs, respectively. The receiver uses this information to perform proper descrambling operations utilizing the corresponding PN sequences. Time division Multiplexing pilot Symbol 2 (TDM2) follows the wide-area and local ID symbols, where TDM2 is periodic, having a 2048 OFDM chip period, and contains two and a fraction periods. The receiver uses TDM2 when determining accurate timing for demodulation.
Following TDM2 are: One wide-area TPC (WTPC) symbol; Five wide-area OIS symbols; Another WTPC; One local TPC (LTPC) symbol; Five local OIS symbols; Another LTPC; and Four data frames follow the first 18 OFDM symbols described above. A data frame is subdivided into a wide-area data portion and a local data portion. The wide-area Data is pre-pended and appended with the wide-area TPC—one on each end. This arrangement is also used for the local data portion. One important aspect is the initial processing of super frame information in order to determine such aspects as the start of a new super frame such that further frame information can be synchronized and determined there from.
There are several problems that are related with conventional pure delayed autocorrelation based timing and frequency acquisition systems. One problem relates to the fact that timing acquisition uses a fixed threshold directly on the delayed correlation estimate to detect a rising and trailing edge of a delayed autocorrelation estimate calculated directly from a hypothesized TDM Pilot 1 waveform. This method suffers from the sensitivity to the variation of noise/interference level such as caused by a tone jammer. There are other variations of the pure autocorrelation based methods which have similar limitations. Another problem is that current frequency acquisition algorithms update a frequency offset during the coarse timing acquisition period which results in at least two drawbacks: First, it impairs the correlation used for timing acquisition; second, it provides degraded frequency estimate which may cause acquisition failure. Another problem relates to large detection delays of conventional systems, resulting in the potential missed processing of the next OFDM symbol.